(1) Field of the Invention
The present invention relates to an apparatus for heat-treating substrates such as semiconductor wafers, glass substrates for photo-masking, and glass substrates for liquid crystal displays, in which the substrates are heated, cooled, or maintained at room temperature. Hereinbelow, the term "heat-treating" substrates is defined to include heating and cooling substrates alike.
(2) Description of the Related Art
A known heating apparatus which is one example of such heat-treating apparatus is disclosed in Japanese Patent Laying-Open Publication No. 1990-300159. In this apparatus, substrates are transported on what is known as a walking transport beam to be placed on and removed from a heating area. The hot plate is surrounded by adiabatic covering, and atmosphere in the hot plate is exhausted through a lateral position of the covering.
The above known apparatus has the following disadvantage.
Proposals have been made in recent years to use a plurality of heat-treating units in order to increase the number of substrates processed per unit time, the heat-treating units being arranged in vertical stages to save installation space of the apparatus. To exhaust gas as in the foregoing known apparatus from the individual heat-treating units arranged in vertical stages requires an increased number of exhaust ducts and the like, which results in a large overall construction of the apparatus.